Process for liquefaction of natural gas

ABSTRACT

A process and system for production of liquefied natural gas (LNG) from natural gas. The natural gas is first partially purified by removal of water and other contaminants, followed by partial chilling to freeze some contaminants and to allow for production of a purge stream to remove other contaminants. These contaminants may be removed from the stream. The process has advantages of low cost and improved removal of contaminants.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Provisional Application No. 61/860,319 filed Jul. 31, 2013, the contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Liquefied natural gas or LNG is natural gas (predominantly methane, CH4) that has been converted to liquid form for ease of storage or transport. The liquefaction process involves removal of certain components, such as dust, acid gases, helium, water, and heavy hydrocarbons, which could cause difficulty downstream in the liquefaction process. The natural gas is condensed into a liquid at ambient pressure. It is typically liquefied at ambient pressure at −101° C. (maximum transport pressure set at around 25 kPa (3.6 psi) and then cooled to −162° C. by using a Joule-Thompson expansion or through the use of a subcooler. By raising the pressure of liquefaction, the latent duty is reduced, improving the efficiency of the liquefaction cycle. LNG achieves a higher reduction in volume than compressed natural gas (CNG) so that the energy density of LNG is about 2.4 times greater than that of CNG and about 60% of that of diesel fuel. This makes LNG cost efficient to transport over long distances where pipelines do not exist.

Specially designed cryogenic sea vessels (LNG carriers) or cryogenic road tankers are used for its transport. The natural gas fed into the LNG plant is treated to remove water, hydrogen sulfide, carbon dioxide and other components that will freeze (such as benzene) under the low temperatures needed for storage or be destructive to the liquefaction facility. Also, hydrocarbons heavier than methane are removed for higher value uses. LNG typically contains more than 90% methane. It also contains small amounts of ethane, propane, butane, some heavier alkanes, and Nitrogen. The purification process can be designed to give almost 100% methane.

The cost of building an LNG liquefaction plant has steadily increased by about five times more as compared to ten years ago, largely due to high raw material prices as well as other factors. Due to these high costs it is desirable to develop more efficient processes and equipment for producing LNG.

Pipeline natural gas typically contains levels of H₂O, CO₂, and other materials which are perceived to require removal prior to liquefaction. Due to their freeze points, during the cooling process, these materials will tend to foul the heat exchangers and lead to blockages. Therefore, in order to solve this problem, the industry will typically use PSA, solvent scrubbers and membranes in order to remove these contaminants prior to liquefaction. While cost effective in large scale units, the additional costs in small scale units tends to drive up the total cost of ownership for the production of LNG. The issue therefore is to extract these contaminants without adding large amounts of capital cost.

SUMMARY OF THE INVENTION

The invention provides a process and a system for producing a LNG product. There are several variations of the process and the system. The process, which uses several purification devices, chillers and a column, includes multiple steps.

In an embodiment of the process, at least a portion of the supply of natural gas is at least partially purified with removal of water and other contaminants. The next step is to feed a stream of LNG as well as the supply of natural gas to a column such as a quench tower. The first section of the process uses latent heat in the LNG to cool the feed incoming natural gas. The first section is run with excess liquid such that the volatile components in the natural gas as condensed and frozen into the cooling liquid.

The chilled gas may be a quantity of the product from the process, liquefied natural gas. A portion of at least one contaminant condenses, solidifies or dissolves. The next step may be to filter out these contaminants followed by further cooling to the necessary temperature to produce LNG. In another embodiment of the invention, solids may end up in the end product.

The process involves liquefaction of natural gas, comprising cooling a natural gas stream to a temperature from about 0° to −100° C. to produce a cooled natural gas stream, sending the cooled natural gas stream to a quench unit, sending a quantity of liquefied natural gas to the quench unit to be combined with the cooled natural gas steam to produce a bottoms stream comprising an intermediate cooled natural gas stream comprising solid impurities and a top stream comprising methane and incondensable impurities, The bottoms stream is then sent to a unit to remove solid impurities to a produce a purified bottoms stream; and the purified bottoms stream is then cooled to produce liquefied natural gas. Prior to the cooling of the natural gas stream, at least a portion of water is removed as well as other contaminants. The natural gas stream may be cooled to a temperature of about −25° to −75° C. or about −55° to −60° C. The solid impurities that are removed include carbon dioxide, C₅ and C₆ hydrocarbons and water. The incondensable impurities are selected from the group consisting of nitrogen, helium, oxygen and hydrogen. Typically, the bottoms stream is at a temperature from about −75° to −120° C. and preferably it is at a temperature from about −90° to −100° C. The invention also comprises a system for producing liquefied natural gas from a supply of natural gas, comprising a device for supplying a stream of natural gas, a separation device to remove water and other impurities from the stream of natural gas to produce a partially dried stream of natural gas, a means for feeding the partially dried stream of natural gas to a chilling device and then to a quench tower, a means for feeding a quantity of a chilled liquid to the quench tower where the chilled liquid and said partially dried stream of natural gas are present in said quench tower and a means to vent a purge stream from said quench tower; a means to transport a combination of the chilled liquid and the partially dried stream of natural gas to a filter unit; a means to transport a purified stream from the filter unit to a chiller to produce liquefied natural gas and a means to the said liquefied natural gas to a storage device. The LNG may then be transported by ship or truck to a customer.

The invention overcomes the issue of fouling or blocking of the heat exchange device used to liquefy the natural gas by undertaking the majority of the cooling duty of the natural gas stream through direct contact with sub cooled LNG or saturated LNG. The natural gas stream is introduced to a quench unit and sending a quantity of liquefied natural gas to the quench unit to be combined with the natural gas stream, a bottoms stream comprising solid impurities in a liquefied hydrocarbon is produced and a top stream comprising methane and incondensable impurities. The bottoms stream is then sent to a unit to remove solid impurities to a produce a purified bottoms stream. The bottoms stream may be cooled to the liquefied natural gas temperature prior to or after purification. Without a reduction in pressure, this cooling of the natural gas will produce a sub cooled liquefied natural gas. A portion of the impurities, such as water, may be removed by first passing the gas through other impurity removal devices in order to enable some of the cooling to be undertaken in an indirect heat exchange device. In such a case, the natural gas stream may be cooled to a temperature of about −25° to −75° C. prior to introduction into the quench device. The solid impurities that are removed include carbon dioxide, C₅ and C₆ hydrocarbons and water. The incondensable impurities are selected from the group consisting of nitrogen, oxygen, helium and hydrogen. Typically, the bottoms stream is at a temperature from about −75° to −120° C. and preferably it is at a temperature from about −90° to −100° C. The top stream may be condensed using either an indirect liquefaction process, such as a heat exchanger, or through direct contact with a portion of the sub cooled liquefied natural gas. Further, by running this top stream liquefied with excess sub cooled natural gas, a feed stream of sub cooled or saturated LNG may be provided to the quench unit. A means to transport a purified stream from the filter unit to a chiller to produce liquefied natural gas and a means to the said liquefied natural gas to a storage device. The LNG may then be transported by ship or truck to a customer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a flow scheme for the production of liquefied natural gas from natural gas through indirect condensation

FIG. 2 illustrates an alternative flow scheme of the production of liquefied natural gas from natural gas through direct condensation

FIG. 3 illustrates a flow scheme in which LNG is generated from the quench condensate

DETAILED DESCRIPTION

The invention solves the problem by undertaking the liquefaction in a way which does not lead to blockage of the condenser. Therefore, contaminants can be introduced into this unit, and then removed as a solid from the resulting LNG. While it is feasible to purify the natural gas to a level that allows for production of LNG, the present invention provides a process to reduce overall costs of production.

Low cost precleaning of the natural gas stream may also take place as needed, reducing the load on the filters. In an embodiment, the design would spray supercooled LNG into the top of a column, and introduce the warmer natural gas feed into the bottom where the supercooled LNG and warmer natural gas feed would mix. While the two fluids would be thermally in equilibrium at the top of the column, at the bottom there may be more than a 38° C. (100° F.) temperature difference in temperature between the liquid and the gas. The thermal equilibrium will occur partway down the column. Given that at the top of the column, liquefaction of the methane rich gas is taking place through contact with a methane rich liquid, the liquid must be at a colder temperature than the gas phase.

This quenching operation at the bottom of the column preferentially extracts the contaminants into the liquid phase where they can be extracted. The lighter components rise up the column, where they in turn condense. Some of the nitrogen, helium, hydrogen, argon and oxygen may exit the top of the column as a small purge stream while some of these gases may form part of the product. A small portion of the methane stream will also escape into the purge stream. All other components, including carbon dioxide and water, are condensed into a ‘dirty’ LNG then passes through a series of filters to be cleaned and then cooled to the necessary storage tank temperature of −161° C. (−257° F.) by using Joule-Thompson expansion or a subcooler. From this tank, liquid is pumped to the top of the column in order to condense the feed. Periodically, the hydrocarbon/carbon dioxide filter cake would be removed and either combusted or used in other applications. The unit therefore would consist of a low cost front end bed (dryer or similar unit), precooler, wash column, filter, subcooler, storage tank and pump.

U.S. Pat. No. 6,637,240 described a method for making nitrogen gas using a tank of liquid nitrogen. Cool air is introduced into the bottom, and liquid nitrogen is introduced at the top. Akin to this unit, the temperature difference at the bottom of the column produces the same sort of cryogenic quench. Further the bottoms (a crude dirty liquid oxygen) is viewed as waste and not subsequently subcooled in order to generate both a product as well as the quenching liquid. In other cryogenic nitrogen patents, the crude liquid oxygen is sub cooled through a Joule Thompson effect and used to provide liquid to the top of the column. However, it achieves this though indirect heat exchange with the top gases and not by direct contact. Due to the higher temperature difference between the freeze point of the solids and the internal surfaces of the column, the solids do not foul the column. It is only with low temperature differential during freezing that the two solids can form a cohesive bond. One key factor is that fouling does not take place in the quench because of the high temperature difference between the solid surfaces and the incoming natural gas. Adhesion during the freezing stage only takes place if there is a low temperature difference. The high temperature difference therefore precludes this adhesion taking place and the formed solids will be washed away with the liquid.

FIG. 3 shows a simplified flow sheet of the process of the present invention. In an example of the process, 11.34 kg (25 lb) mol/hr of natural gas is scrubbed of a portion of the water (but not to LNG specifications) and introduced into a prechiller cooling it to −58° C. This cooled natural gas is then introduced into a quencher and cooled using 17 kg (37.5 lb) mol/hr of sub cooled LNG that is at about −162° C., the standard temperature for LNG. The majority of the feed stream is condensed. The quencher is fed from 62 which is a partial condenser. The quencher itself is fed with a saturated liquid from the partial condenser 62. While there may be one column, in the embodiment shown in FIG. 3, there are two columns.

A purge stream containing the incondensable contaminants exits from the top of the column as well as a portion of the methane. A bottom stream at approximately −100° C. contains all of the impurities left by the feed scrubber. Due to the cold temperature, these are in the solid form. A simple mesh filter would then be used to remove these particles, and the liquid would then be further chilled to −161° C. (−257° F.) for storage. During operation, subcooled LNG would be extracted from the storage tank and used in the quench chiller. Simulations suggested that a simple closed nitrogen refrigeration cycle would provide sufficient cooling for this process using 309 hp of power in order to condense 9071 kg (10 tons)/day of LNG. Rather than requiring steam, solvents and other front end purification processes, this unit would run using higher power, a quench tower and a simple filtration device.

FIG. 1 presents one embodiment of the invention which does not generate LNG though direct condensation, nor does it use the bottoms liquid as a source of the product. A natural gas stream 2 is introduced into a simple PSA 6 wherein a portion of the water is removed producing a dry natural gas stream 8 with a dew point below −50° C. and a purge stream 10 containing the removed moisture. Stream 8 is then cooled to −50° C. in an indirect heat exchanger 12 to produce a chilled dry natural gas stream 14. Stream 14 is then introduced into the quench column 16 wherein it is cooled to −100° C. through direct contact with a primary cooling liquid and a substantial portion of the heavy components within stream 14 condense and solidify into the dirty heavy bottom liquid 18. A portion of the primary cooling liquid is evaporated during this process and mixes with the light components of stream 14 and exits the quench column 16 as the chilled medium gas 20. Stream 20 is then further cooled through direct contact in a secondary cooler 22 with a secondary cooling liquid, removing further impurities such as C₂ and C₃ to producing a medium bottom liquid 24 which is fed to the quench column 16 to provide a portion of the primary cooling liquid. The solids in stream 18 are removed through the use of a filter 28 producing a heavy liquid 30 and a solid waste stream 32. Stream 30 is then raised in pressure in recycle pump 34 and fed as heavy cooling liquid 36 to quench column 16 where it forms part of the primary cooling fluid. The remaining uncondensed portion of stream 20 exits the secondary cooler 22 as a light chilled gas 26 and is fully condensed in a liquefier 38 and fed as a light liquid stream 40 to a storage tank 42. When required, a draw stream 44 is removed from tank 42, reduced in pressure by regulator 46 and delivered as liquefied natural gas 48. Meanwhile, a light liquid draw stream 50 is withdrawn from tank 42, raised in pressure in pump 52 in order to be fed to secondary cooler 22 as a light cooling liquid 54. The LNG may be transported by ship, truck or other transport means.

FIG. 2 is an example of the process which generates LNG though direct condensation, but does not uses the bottoms liquid as a source of the product. In this example, A natural gas stream 2 is introduced into a simple PSA 6 wherein a portion of the water is removed producing a dry natural gas stream 8 with a dew point below −50° C. and a purge stream 10 containing the removed moisture. Stream 8 is then cooled to −50° C. in an indirect heat exchanger 12 to produce a chilled dry natural gas stream 14. Stream 14 is then introduced into the quench column 16 wherein it is cooled to −100° C. through direct contact with a primary cooling liquid and a substantial portion of the heavy components within stream 14 condense and solidify into the dirty heavy bottom liquid 18. A portion of the primary cooling liquid is evaporated during this process and mixes with the light components of stream 14 and exits the quench column 16 as the chilled medium gas 20. Stream 20 is then further cooled through direct contact in a secondary cooler 22 with a secondary cooling liquid, removing further impurities such as C₂ and C₃ to producing a medium bottom liquid 24 which is fed to the quench column 16 to provide a portion of the primary cooling liquid. The solids in stream 18 are removed through the use of a filter 28 producing a heavy liquid 30 and a solid waste stream 32. Stream 30 is then raised in pressure in recycle pump 34 and fed as heavy cooling liquid 36 to quench column 16 where it forms part of the primary cooling fluid. The remaining uncondensed portion of stream 20 exits the secondary cooler 22 as a light chilled gas 26 and is fully condensed in a liquefier 38 and fed as a light liquid stream 40 to a storage tank 42. When required, a draw stream 44 is removed from tank 42, reduced in pressure by regulator 46 and delivered as liquefied natural gas 48. Meanwhile, a light liquid draw stream 50 is withdrawn from tank 42, raised in pressure in pump 52 in order to be fed to secondary cooler 22 as a light cooling liquid 54. a secondary light liquid draw stream 56 is removed from tank 42, raised in pressure in pump 58 to form light condensing fluid 60 and introduced into direct condenser 62. The light chilled gas 26 is also introduced into the direct condenser 62 where the majority of the stream is condensed forming light condensate 64. Stream 64 is cooled even further in exchanger 66 to form stream 40. Any incondensable gases leave direct condenser 62 as purge 68.

FIG. 3 does generate LNG though direct condensation and uses the bottoms liquid as a source of the product. In this example, natural gas stream 2 is introduced into a simple PSA 6 wherein a portion of the water is removed producing a dry natural gas stream 8 with a dew point below −50° C. and a purge stream 10 containing the removed moisture. Stream 8 is then cooled to −50° C. in an indirect heat exchanger 12 to produce a chilled dry natural gas stream 14. Stream 14 is then introduced into the quench column 16 wherein it is cooled to −100° C. through direct contact with a primary cooling liquid and a substantial portion of the heavy components within stream 14 condense and solidify into the dirty heavy bottom liquid 18. A portion of the cooling is evaporated during this process and mixes with the light components of stream 14 and exits the quench column 16 as the chilled medium gas 20. Stream 20 chilled medium gas 20 is instead sent to direct condenser 62 to produce the medium bottom liquid 24. Medium bottom liquid 24 which is fed to the quench column 16 to provide a portion of the primary cooling liquid. The solids in stream 18 are removed through the use of a filter 28 producing a heavy liquid 30 and a solid waste stream 32. Heavy liquid stream 30 is cooled in exchanger 66 to form stream 40 and be fed to tank 42. The remaining uncondensed portion of stream 20 exits the secondary cooler 22 as a light chilled gas 26 and is fully condensed in a liquefier 38 and fed as a light liquid stream 40 to a storage tank 42. When required, a draw stream 44 is removed from tank 42, reduced in pressure by regulator 46 and delivered as liquefied natural gas 48. Meanwhile, a light liquid draw stream 50 is withdrawn from tank 42, raised in pressure in pump 52 in order to be fed to secondary cooler 22 as a light cooling liquid 54.

Those experienced in the art will recognize that columns 62, 22 and 16, while shown as separate units could be combined into one single column or their roles could be split across multiple different columns.

SPECIFIC EMBODIMENTS

While the following is described in conjunction with specific embodiments, it will be understood that this description is intended to illustrate and not limit the scope of the preceding description and the appended claims.

A first embodiment of the invention is a process for liquefaction of natural gas, the process comprising a) sending the cooled natural gas stream to a quench unit; b) sending a quantity of liquefied natural gas to the quench unit to be combined with the cooled natural gas steam to produce a bottoms stream comprising a liquid stream comprising solid impurities and a top stream comprising methane and incondensable impurities; and c) cooling the bottoms stream to produce liquefied natural gas. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the natural gas stream is cooled to a temperature from about 0° to −100° C. to produce a cooled natural gas stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising sending the bottoms stream to a unit to remove the solid impurities to a produce a purified bottoms stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein a portion of the bottom stream is returned to the quench unit. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein before the natural gas stream is cooled, at least a portion of water within the natural gas stream is removed. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the natural gas stream is cooled to a temperature of about −25° to −75° C. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the natural gas stream is cooled to a temperature of about −55° to −60° C. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the solid impurities are selected from the group consisting of carbon dioxide, C₅ and C₆ hydrocarbons and water. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the incondensable impurities are selected from the group consisting of nitrogen, oxygen and hydrogen. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the bottoms stream is at a temperature from about −75° to −120° C. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the bottoms stream is at a temperature from about −90° to −100° C.

A second embodiment of the invention is a system for producing liquefied natural gas from a supply of natural gas, comprising a) a device for supplying a stream of natural gas; b) a separation device to remove water from the stream of natural gas to produce a partially dried stream of natural gas; c) a means for feeding the partially dried stream of natural gas to a chilling device and then to a column; d) a means for feeding a quantity of a chilled liquid to the column where the chilled liquid and the partially dried stream of natural gas are present in the column and a means to vent a purge stream from the quench tower; e) a means to transport a combination of the chilled liquid and the partially dried stream of natural gas to a particle removal unit; f) a means to transport a purified stream from the filter unit to a chiller to produce liquefied natural gas; and g) a means to send the liquefied natural gas to a storage device. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the separation device removes additional contaminants from the stream of natural gas. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the column is a quench tower. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the chilled liquid is a quantity of liquefied natural gas.

Without further elaboration, it is believed that using the preceding description that one skilled in the art can utilize the present invention to its fullest extent and easily ascertain the essential characteristics of this invention, without departing from the spirit and scope thereof, to make various changes and modifications of the invention and to adapt it to various usages and conditions. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limiting the remainder of the disclosure in any way whatsoever, and that it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated. 

1. A process for liquefaction of a hydrocarbon stream, said process comprising: a) sending a cooled hydrocarbon stream to a quench unit; and b) sending a quantity of a liquefied hydrocarbon stream to said quench unit to be combined with said cooled hydrocarbon steam to produce a bottoms stream comprising a liquid stream comprising solid impurities and a top stream comprising methane and incondensable impurities.
 2. The process of claim 1 wherein said liquefied hydrocarbon stream is at least 15 degrees C. cooler than a freeze point of solid impurities in the cooled hydrocarbon stream.
 3. The process of claim 1 wherein the liquefied hydrocarbon is relatively evenly distributed across the quench unit.
 4. The process of claim 1 wherein a portion of said bottom stream is used to produce liquefied natural gas.
 5. The process of claim 1 further comprising sending said bottoms stream to a unit to remove said solid impurities to a produce a purified bottoms stream.
 6. The process of claim 1 wherein a portion of said bottom stream is returned to said quench unit.
 7. The process of claim 1 wherein before said natural gas stream is cooled, at least a portion of water within said natural gas stream is removed.
 8. The process of claim 1 wherein said natural gas stream is cooled to a temperature of about −25° to −75° C.
 9. The process of claim 1 wherein said natural gas stream is cooled to a temperature of about −55° to −60° C.
 10. The process of claim 1 wherein said solid impurities are selected from the group consisting of carbon dioxide, C₅ and C₆ hydrocarbons and water.
 11. The process of claim 1 wherein said incondensable impurities are selected from the group consisting of nitrogen, oxygen and hydrogen.
 12. The process of claim 1 wherein said bottoms stream is at a temperature from about −75° to −120° C.
 13. The process of claim 1 wherein said bottoms stream is at a temperature from about
 14. A system for producing liquefied natural gas from a supply of natural gas, comprising: a) a device for supplying a stream of natural gas; b) a separation device to remove water from said stream of natural gas to produce a partially dried stream of natural gas; c) a means for feeding said partially dried stream of natural gas to a chilling device and then to a column; d) a means for feeding a quantity of a chilled liquid to said column where said chilled liquid and said partially dried stream of natural gas are present in said column and a means to vent a purge stream from said quench tower; e) a means to transport a combination of said chilled liquid and said partially dried stream of natural gas to a particle removal unit; f) a means to transport a purified stream from said filter unit to a chiller to produce liquefied natural gas; and g) a means to send said liquefied natural gas to a storage device.
 15. The system of claim 14 wherein said separation device removes additional contaminants from said stream of natural gas.
 16. The system of claim 14 wherein said column is a quench tower.
 17. The system of claim 14 wherein said chilled liquid is a quantity of liquefied natural gas. 